We are seeing a number of crashes in SMM, when VT-d is enabled while
'Legacy USB support' is enabled in various BIOSes.

The BIOS is supposed to indicate which addresses it uses for DMA in a
special ACPI table ("RMRR"), so that we can punch a hole for it when we
set up the IOMMU.

The problem is, as usual, that BIOS engineers are totally incompetent.
They write code which will crash if the DMA goes AWOL, and then they
either neglect to provide an RMRR table at all, or they put the wrong
addresses in it. And of course they don't do _any_ QA, since that would
take too much time away from their crack-smoking habit.

The real fix, of course, is for consumers to refuse to buy motherboards
which only have closed-source firmware available. If we had _open_
firmware, bugs like this would be easy to fix.

Since that's something I can only dream about, this patch implements an
alternative -- ensuring that the USB controllers are handed off from the
BIOS and quiesced _before_ the IOMMU is initialised. That would have
been a much better design than this RMRR nonsense in the first place, of
course. The bootloader has no business doing DMA after the OS has booted
anyway.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>